The invention relates generally to the generating of electric power in space and more specifically concerns the generation of electric power from solar energy in space.
Prior methods of producing electric power from solar energy in space are classified as follows: photovoltaic (solar cells), thermoelectric, and plasma magnetohydrodynamic (MHD) generators. There also has been a proposal for a liquid metal magnetrohydrogynamic (LMMHD) generator driven by a nuclear reactor in space.
The photovoltaic generation of electric power has been utilized for many years with solar cells on various spacecraft. The efficiency of the solar cells has been improved considerably due to intensive research efforts. However, the solar cells are difficult to operate at temperatures greater than 200.degree. C. due to sharp decreases of efficiency and useful life. Consequently, solar-cell panels require a large area, thus the unit cost for electric power from solar cells is very high. Also, the cost and technology required for lifting the large payload into earth orbit becomes prohibitive. For example, to produce 25 kW of electric power from solar cells with 10 percent efficiency, the effective area of the cells covers approximately 180 m.sup.2. The electrical circuit for power collection also becomes a costly task for such a large but diffuse power source in space. Although improvements of efficiency and operating temperature for solar cells will reduce the difficulties somewhat, the demand for high power levels in space will undoubtedly increase in the future and the difficulties to satisfy this demand with solar cells will remain.
Thermal power production from solar radiation in space has been extensively investigated since this method requires minimal modifications of the well developed technology for conventional steam power plants on earth. However, the efficiency, typically less than 35 percent, of this method is limited by the rather low temperature (below 1000K) of steam generated with solar energy.
On the other hand, plasma MHD generators operated at temperatures greater than 2000K gives a high efficiency for electric power production. However, continuous operation at such high temperatures results in severe material problems yet to be solved. The maximum duty cycle tested for a high temperature plasma MHD lasted only a few days with coal gas as the working fluid.
A prior art liquid metal MHD generator (FIG. 1) was originally proposed as part of a space power system using a nuclear reactor. Two metals, cesium and lithium, were considered as the working fluids of the generator. The cesium (Cs), leaving a radiator 11 as a condensate, is pumped by a pump 12 through a regenerative heat exchager (not shown) to a nozzle 13 where it vaporizes as it comes in contact with the liquid metal, lithium (Li), from the liquid loop. The cesium accelerates the lithium in the nozzle, thus imparting an increased kinetic energy to the liquid lithium; the cesium is separated from the lithium in a separator 14 and then passes back to the radiator. The lithium leaves the separator at a relatively high velocity, approximately 150 m/s and flows through an MHD generator 15. The cooled Li is passed through a diffuser 16 and then reheated in a heat source (nuclear reactor) 17 and then pumped back to nozzle 13.
The disadvantages of this device--a fixed and high operating temperature range (&gt;1400.degree. C.) and the difficulty of handling of the liquid flow in the MHD channel; have been alleviated by the adoption of a two-phase generator cycle. The basic idea was to utilize the fact that a two-phase mixture is a compressible fluid and thus is an effective thermodynamic working fluid that could be expanded directly through the MHD generator like a gas expanding through a turbine from which electric power is extracted (FIG. 2). The mixture as it leaves the MHD generator 20 is further expanded in a nozzle 21 to increase its kinetic energy and is then sent to a separator 22. There the liquid metal is separated from the gas and is returned via a diffuser 23 through the heat source 24 to the mixer 25. The gaseous working fluid is then handled as in a normal Brayton cycle; it is passed through the regenerative heat exchanger 26 to the heat sink 27 and is then compressed by a compressor 28 and sent back to the mixer 25 via the heat source 24. A gas source 29 supplies the gas needed to startup and for replenishing the gas lost in the operation. The gaseous component is the thermodynamic working fluid, and the liquid metal, which remains in the closed liquid loop, is the electrodynamic working fluid. The heat sources considered by researchers for use with the LLMHD generator shown in FIG. 2 have been fossil combustion, high temperature gas cooled nuclear reactor (HTGCR), fusion reactors, and liquid metal fast breeder reactors.
It is an object of this invention to provide a means for generating electric power in space.
Another object of this invention is to provide a new means for generating electric power from solar energy.
A further object of this invention is to utilize solar heat as the heat source for MHD generators.
Still another object of this invention is to provide a solar driven LMMHD generator.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.